My board is MSI P6NGM-L ,for about a year using my desktop,sudenly set on stand by mode,how can i fix it?or reset it?

During the boot process you will see on the screen to press a certain key to enter setup
Press and hold that key during the boot up process to enter BIOS you can load failsafe defaults or load optimized defaults ,press escape then press f10 to save to cmos to restart
or you might try
During the boot process tap the f8 button three times this will boot into safe mode where the only things
working will be your mouse and keyboard enabling you to attempt to repair your computer
hope this helps